C.Scale Weather Service

Historical Weather Data: TMYx EPW Files

What Is a TMYx File?

A Typical Meteorological Year extended (TMYx) file is a single-year dataset of hourly weather observations designed to represent the long-run average climate of a location. Each hour in the file is drawn from a real observed year, selected statistically to best represent typical conditions over a multi-decade baseline period. The result is a synthetic but physically realistic year that neither anomalously hot nor cold — it represents the middle of the distribution.

TMYx files are the standard input for building energy simulation tools including EnergyPlus, OpenStudio, DesignBuilder, IES-VE, and eQUEST. A simulation run against a TMYx file answers the question: "How does this building perform in a typical year at this location?"

"TMY datasets are not appropriate for simulating extreme or atypical weather conditions. They represent typical rather than extreme conditions and should not be used to size systems for worst-case loads."
— U.S. Department of Energy, EnergyPlus Engineering Reference

How TMYx Differs from TMY3 and TMY2

Format Baseline period Solar data source Coverage Status
TMY2 1961–1990 NSRDB ground stations 239 US sites Superseded
TMY3 1976–2005 NSRDB ground stations 1,020 US/territory sites Superseded
TMYx 1987–present (varies by vintage) ERA5 reanalysis (global satellite) ~17,000+ global WMO stations Current

The shift from ground-station solar data (NSRDB) to ERA5 satellite-derived reanalysis is the most significant methodological update in TMYx. ERA5 provides consistent, worldwide coverage — making TMYx suitable for global simulation in a way that earlier formats were not. TMY2 and TMY3 were limited to US and territory stations because the NSRDB only covered North America.

"TMYx files are an update to the TMY family of files… One major update in TMYx files is the use of the ERA5 satellite-derived dataset for solar radiation, instead of the National Solar Radiation Database (NSRDB). ERA5 contains worldwide satellite data, which is more suitable than US-concentrated data for calculating other climate variables."
— Lawrie & Crawley (2022), Development of Global Typical Meteorological Years (TMYx)

Vintage: Why 2011–2025 Matters

TMYx files are released in multiple vintages. This service provides only the 2011–2025 vintage — the most recent available — which covers observations from 2011 through 2025.

Older vintages (e.g. 2007–2021) embed a baseline climate that is systematically cooler than current conditions, because they include more years from before the acceleration of warming observed since ~2015. Using a 2011–2025 baseline ensures that simulations reflect the climate buildings are actually operating in today.

The vintage is returned in every API response in the vintage field, so downstream tools always know the exact observation period.

Coverage

TMYx stations are derived from the NOAA Integrated Surface Database (ISD), which collects observations from WMO (World Meteorological Organization) weather stations worldwide. This service loads ~17,000 WMO stations globally, covering every inhabited continent.

Nearest-station lookup finds the closest WMO station to a given latitude and longitude. The distance to the matched station is returned in every API response — users should consider this distance when assessing how representative the file is for their specific site, particularly in regions with sparse station coverage.

File Format

TMYx files follow the EnergyPlus Weather (EPW) format: a plain-text comma-separated file with 8,760 rows of hourly data (one calendar year, excluding leap day), preceded by a location header with station metadata. Fields include:

EPW files are directly consumable by EnergyPlus and most major simulation tools without conversion.

Data Source and Attribution

TMYx files are produced and maintained by Linda K. Lawrie and Drury B. Crawley and distributed through climate.onebuilding.org.

License: Creative Commons Attribution 4.0 International (CC BY 4.0)

Required attribution:

Lawrie, L.K., Crawley, D.B. (2022). Development of Global Typical Meteorological Years (TMYx). http://climate.onebuilding.org

When using TMYx data obtained through this service, include the above attribution in any published work, report, or software that relies on these files.

Known Limitations

TMYx files represent typical conditions, not extremes.
They are constructed to reproduce the average annual energy balance of a location. The statistical selection process filters out anomalously hot or cold years, which means extreme events — heat waves, polar vortex intrusions, drought — are underrepresented by design. TMYx files are appropriate for annual energy use estimates, not for peak load sizing under worst-case conditions.

TMYx is a historical snapshot, not a forecast.
A 2011–2025 TMYx file reflects observed past climate. It does not capture the warming projected over the lifetime of a building designed today. For climate-resilient design, pair TMYx simulations with future climate projections (see Future Climate Data).

Station density varies by region.
High-density station networks in the US, Europe, and East Asia provide good spatial coverage. Remote regions — high-latitude areas, central Africa, small islands — may have the nearest station hundreds of kilometres away. The API returns the distance to the matched station; treat files with distances above ~100 km with appropriate caution.

TMYx construction can smooth out climate trends within the baseline period.
Because the statistical selection algorithm chooses individual months from across the full baseline period to assemble the synthetic year, a 2011–2025 TMYx file may not fully reflect the warming that occurred at the end of that period. The file represents the average of 2011–2025, not the conditions of 2024 or 2025 specifically.

References